“Intermittent Fasting” refers to a dieting strategy that restricts food intake for extended periods of time at regular intervals. There are several common models, but one of the most common allows for 24 hours of normal eating followed by 24 hours of fasting.
Hopeful ears perk up at bold claims of faster fat loss, better muscle maintenance, clearer thinking and disease prevention. I plowed through 15+ studies (linked below) in search of some half-way meaningful scientific consensus, and here’s what they seem to agree on:
- Intermittent fasting does NOT make you lose more weight than regular caloric restriction of the same amount. That just means that energy balance is still king when it comes to weight loss, as it always will be. Intermittent fasting may have some benefits, but if your caloric balance is the same, your overall rate of weight loss over time will be the same. However,
- Voluntary caloric consumption tends to decrease with intermittent fasting. Meaning that when subjects ate what they wanted for 24 hours and then fasted for 24 hours, their total energy consumption was less than subjects that ate as they pleased. So that’s kinda cool. “But how will I train on an empty stomach??” good point, sir. Or ma’m. Whoever you are. Also know:
- Performance during anaerobic exercises like lifting and sprinting will probably suffer while fasting. This is important to note because in order to make progress you need to be able to apply overload, meaning you must do more than you have previously done in some way. If you can’t because you have no energy and are day dreaming about maple bacon donuts, making progress is going to be very difficult.
One study also showed that fasting had a negative effect on reaction time in athletes. However, in another study done on resistance trained males, muscle mass was maintained just as well over an 8 week period of intermittent fasting as it was during normal dieting. There was a decrease in testosterone in the intermittent fasting group. For those simply interested in general health benefits though:
- Insulin sensitivity seems to be improved by intermittent fasting. If you’re at risk for cardiovascular disease this could potentially have some valuable application (talk to your doc). However, if you’re young-ish and pretty healthy, your insulin sensitivity is probably groovy, so there’s no need to suffer 24 famished hours on that account. Additionally,
- Studies on mice have shown promising results for preventing Alzheimer’s and Parkinson’s disease but these are far from conclusive (according to a review of the available literature published October of 2017) and more human studies, especially long term ones, need to be completed before forming hasty conclusions. Humans and mice do have a few differences after all.
Intermittent fasting isn’t the devil and might have some useful application for specific populations, such as those at risk for heart disease. Additionally, in my experience with fasting and with clients who have tried it, it can be a valuable psychological exercise for learning how to self regulate while hungry, as well as simple way to restrict calories without the effort required to track calories accurately, follow a very specific meal plan.
For those of you interested in performance, I would not recommend intermittent fasting. I would recommend well fed training and special attention to your macro nutrients and nutrient timing that will optimize your athleticism.
For those interested in finding a simple way of reducing calories for weight loss or health purposes without having to think about it too much, intermittent fasting can be an effective option, although I would not recommend fasting on the days you do resistance training. If you lift 3 days a week, maybe fast and walk on 3 different days of the week.
As always, I hope this review was helpful to all of you! Here’s a list of the summarized studies below for those interested:
Year-long weight loss treatment for obese patients with type II diabetes: Does including an intermittent very-low-calorie diet improve outcome?
“conclusions: The intermittent VLCD improved weight loss and glycemic control, but these effects were quite modest and do not appear to justify the clinical use of an intermittent VLCD. Moreover, lengthening treatment to a full year did not prevent relapse. Thus, further research is needed to develop a successful approach to long-term weight control.”
The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women
“IER (Intermittent energy restriction) is as effective as CER with regard to weight loss, insulin sensitivity and other health biomarkers, and may be offered as an alternative equivalent to CER for weight loss and reducing disease risk.”
Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?
“These findings suggest that these diets are equally as effective in decreasing body weight and fat mass, although intermittent CR may be more effective for the retention of lean mass.”
(*** I dug into the “lean mass retention” portion of the study and found that this is a huge “may be.” They compared two different studies that used different diets and different ways of measuring lean mass, so in my opinion the information is not reasonably comparable. )
The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women (3 month weight loss, one month maintenance).
“Insulin resistance reduced with the Intermittent Energy and Carbohydrate Restricted diets and the IECR+PF diet. Reductions with the IECR diets were significantly greater compared with the DER (Daily Energy Restriction) diet. Both IECR groups had greater reductions in body fat compared with the DER group”
Time-restricted feeding in young men performing resistance training: A randomized controlled trial
“Overall, TRF (time restricted feeding) reduced energy intake and did not adversely affect lean mass retention or muscular improvements with short-term RT in young males.”
Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans
“These findings suggest that the combination produces superior changes in body weight, body composition, and lipid indicators of heart disease risk, when compared to individual treatments.”
Short- and long-term effects of continuous versus intermittent restrictive diet approaches on body composition and the metabolic profile in overweight and obese postmenopausal women: a pilot study
“The ID resulted in similar short- and long-term changes in body composition and metabolic profile compared with a CD. Most improvements occurred during the first 5 weeks of treatment in both interventions.”
Effects of Ramadan Intermittent Fasting on Sports Performance and Training: A Review
“The small numbers of well-controlled studies that have examined the effects of Ramadan on athletic performance suggest that few aspects of physical fitness are negatively affected, and only modest decrements are observed. Whereas subjective feelings of fatigue and other mood indicators are often cited as implying additional stress on the athlete throughout Ramadan, most studies show these measures may not be reflected in decreases in performance.”
Caloric restriction and intermittent fasting: Two potential diets for successful brain aging
Three Days of Intermittent Fasting: Repeated-Sprint Performance Decreased by Vertical-Stiffness Impairment
Intermittent fasting showed acute decrease in anaerobic performance
“There are multiple interactive pathways and molecular mechanisms by which CR and IF benefit neurons including those involving insulin-like signaling, FoxO transcription factors, sirtuins and peroxisome proliferator-activated receptors.”
Effect of intermittent fasting and refeeding on insulin action in healthy men
“This experiment is the first in humans to show that intermittent fasting increases insulin-mediated glucose uptake rates, and the findings are compatible with the thrifty gene concept.”
Repeated Sprints in Fasted State Impair Reaction Time Performance
“Conclusion: Simple and complex reaction times during the RTI test were negatively affected by the 3d-IF after 2 bouts of intensive repeated sprints.”
Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings
“Results reveal superior decreases in body weight by Caloric Restriction vs Intermittent Fasting/Alternate Day Fasting regimens, yet comparable reductions in visceral fat mass, fasting insulin, and insulin resistance. None of the interventions produced clinically meaningful reductions in glucose concentrations.”
Impact of intermittent fasting on health and disease processes (Alzheimers, parkinsons)
“In laboratory rats and mice IF and PF have profound beneficial effects on many different indices of health and, importantly, can counteract disease processes and improve functional outcome in experimental models of a wide range of age-related disorders including diabetes, cardiovascular disease, cancers and neurological disorders such as Alzheimer’s disease Parkinson’s disease and stroke.”
Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males
“Our results suggest that an intermittent fasting program in which all calories are consumed in an 8-h window each day, in conjunction with resistance training, could improve some health-related biomarkers, decrease fat mass, and maintain muscle mass in resistance-trained males.”
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